The present invention relates to a self venting septum for use in a liquid filtration system. By "septum" it is meant a filter cartridge of a generally tubular configuration having a cylindrical filtration screen thereon through which liquid passes from the exterior to the interior. The invention is particularly concerned with a septum having means for self venting whereby air trapped within the septum as a vessel in which the septum is located is filled with liquid can vent to the exterior. The ability of the septum to self vent expose substantially the entire vertical length of the septum to the flow of liquids therethrough. This is particularly important during the process of pre-coating the septum filtration screens with a medium to augment separation effectiveness.
A well known means of effectively purifying liquids is by passing them through filter screens which have been pre-coated with a layer of filter enhancing material, such as, ion exchange resin particles or other pre-coat medium which may include also diatomaceous earth. In the typical system, a plurality of septums are spaced within a filter tank. The filter tank includes an upper influent compartment and a lower filtrate compartment separated by a generally horizontal divider. The septums or filtration cartridges are vertically supported to the divider plate and extend upwardly within the influent compartment. Each septum includes a cylindrical perforated core element. Covering the perforated core is a filtration screen. The screen is usually of multi-layers with a core screen adjacent to the perforated tubular core member and with at least one finer mesh screen on the outside of the core screen.
To increase the filtration effectiveness a standard procedure is to pre-coat the filter cartridge using a water slurry of pre-coat medium. To do this the water slurry having the pre-coat medium is passed into the tank influent compartment. As the water, having the pre-coat medium suspended therein, enters the influent compartment the level rises and begins to infiltrate the septums, that is, to pass through the fine mesh screens on the exterior of the septums. As the water level rises on the exterior of the septums it also rises on the interior but at a slower rate because of the bubble point pressure of the wetted septum surfaces. This filling procedure, wherein water is added until it extends over the top of the septums can result in air being trapped within the septums. If air is trapped and not released, no water flows through the upper portion of the septums because of the trapped air therein. During fill of the vessel influent compartment, air entrapped in the septum is being forced out by the incoming water and is conversely being resisted to flow out as a result of the bubble point pressure of the wetted septum surface. Thus an equilibrium pressure is formed in which the driving force for the air to flow out of the septum is equalized by the bubble point pressure of the wetted septum. The bubble point of various types of filter medium is frequently expressed in terms of inches of water column pressure necessary to force the first bubble of air through the wetted medium surface. For example, if the septum is made from media with a bubble point pressure of two inch water column, then it follows that when an equilibrium is reached there will remain a two inch air space in the top of each septum. This is significant since the pre-coat material is intended to form a uniform pre-coat on the entire septum surface. A uniform pre-coat however cannot be established at the top two inches of each septum since this space is filled with air.
When the pre-coating is completed, the vessel is closed off and pressurized, thereby decreasing the volume of air in the top of each septum, due to compressibility of the air, and during operation a portion of the top 2 inches of each septum will allow raw untreated water to flow through the septum without flowing through the pre-coat, such as without flowing through an ionic exchange media. The result is untreated water entering into the vessel lower filtrate compartment.
Others have attacked this problem and for background information relating to this disclosure, U.S. Pat. Nos. 3,680,700 that issued Aug. 1, 1972 entitled "Method of Pre-coating Filter Cartridges", and U.S. Pat. No. 3,779,386 that issued Dec. 18, 1973 entitled "Filter Cartridge" are helpful. U.S. Pat. No. 4,293,414 that issued on Oct. 6, 1981 and entitled "Slotted Sheet Filter Element" shows a cylindrical filter element having a function similar to the apparatus of the present disclosure but employs a discontinuously slotted sheet in which each of the slots have a particle retention capability less than about 50 microns. These three issued patents are incorporated herein by reference as each discloses and describes in substantial detail the employment of filtration systems including vessels having a plate dividing the interior into an upper influent compartment and a lower filtrate compartment and in which a plurality of spaced apart vertical cylindrical septums are employed for filtration. Further, these patents disclose the concept of pre-coating the septums for enhanced filtration and/or water treatment action.
For further background information relating to the subject of this disclosure, see the following United States patents:
______________________________________ U.S. Pat. No. INVENTOR TITLE ______________________________________ 3373104 Ryan Method of Cleaning Filter Tank 3666097 Ryan Method and Apparatus For Cleaning A Filter Cartridge 4267039 Ryan Fluid Filtration Method and Apparatus 4405466 Giannelli et al Backwash Method and Apparatus 4904380 Bhanot et al Precoat Filter Assembly 4973404 Weber et al Micro/Ultra Filtration System 5399265 Nehis Filter Septum ______________________________________